---
title: "Custom Scorers | Evals"
---

# Custom scorers

Mastra provides a unified `createScorer` factory that allows you to build custom evaluation logic using either JavaScript functions or LLM-based prompt objects for each step. This flexibility lets you choose the best approach for each part of your evaluation pipeline.

### The Four-Step Pipeline

All scorers in Mastra follow a consistent four-step evaluation pipeline:

1. **preprocess** (optional): Prepare or transform input/output data
2. **analyze** (optional): Perform evaluation analysis and gather insights
3. **generateScore** (required): Convert analysis into a numerical score
4. **generateReason** (optional): Generate human-readable explanations

Each step can use either **functions** or **prompt objects** (LLM-based evaluation), giving you the flexibility to combine deterministic algorithms with AI judgment as needed.

### Functions vs Prompt Objects

**Functions** use JavaScript for deterministic logic. They're ideal for:

- Algorithmic evaluations with clear criteria
- Performance-critical scenarios
- Integration with existing libraries
- Consistent, reproducible results

**Prompt Objects** use LLMs as judges for evaluation. They're perfect for:

- Subjective evaluations requiring human-like judgment
- Complex criteria difficult to code algorithmically
- Natural language understanding tasks
- Nuanced context evaluation

You can mix and match approaches within a single scorer - for example, use a function for preprocessing data and an LLM for analyzing quality.

### Initializing a Scorer

Every scorer starts with the `createScorer` factory function, which requires a name and description, and optionally accepts a type specification and judge configuration.

```typescript
import { createScorer } from '@mastra/core/evals';

const glutenCheckerScorer = createScorer({
  name: 'Gluten Checker',
  description: 'Check if recipes contain gluten ingredients',
  judge: {                    // Optional: for prompt object steps
    model: 'openai/gpt-5.1',
    instructions: 'You are a Chef that identifies if recipes contain gluten.'
  }
})
// Chain step methods here
.preprocess(...)
.analyze(...)
.generateScore(...)
.generateReason(...)
```

The judge configuration is only needed if you plan to use prompt objects in any step. Individual steps can override this default configuration with their own judge settings.

#### Agent Type for Agent Evaluation

For type safety and compatibility with both live agent scoring and trace scoring, use `type: 'agent'` when creating scorers for agent evaluation. This allows you to use the same scorer for an agent and also use it to score traces:

```typescript
const myScorer = createScorer({
  // ...
  type: "agent", // Automatically handles agent input/output types
}).generateScore(({ run, results }) => {
  // run.output is automatically typed as ScorerRunOutputForAgent
  // run.input is automatically typed as ScorerRunInputForAgent
});
```

### Step-by-Step Breakdown

#### preprocess Step (Optional)

Prepares input/output data when you need to extract specific elements, filter content, or transform complex data structures.

**Functions:** `({ run, results }) => any`

```typescript
const glutenCheckerScorer = createScorer(...)
.preprocess(({ run }) => {
  // Extract and clean recipe text
  const recipeText = run.output.text.toLowerCase();
  const wordCount = recipeText.split(' ').length;

  return {
    recipeText,
    wordCount,
    hasCommonGlutenWords: /flour|wheat|bread|pasta/.test(recipeText)
  };
})
```

**Prompt Objects:** Use `description`, `outputSchema`, and `createPrompt` to structure LLM-based preprocessing.

```typescript
const glutenCheckerScorer = createScorer(...)
.preprocess({
  description: 'Extract ingredients from the recipe',
  outputSchema: z.object({
    ingredients: z.array(z.string()),
    cookingMethods: z.array(z.string())
  }),
  createPrompt: ({ run }) => `
    Extract all ingredients and cooking methods from this recipe:
    ${run.output.text}

    Return JSON with ingredients and cookingMethods arrays.
  `
})
```

**Data Flow:** Results are available to subsequent steps as `results.preprocessStepResult`

#### analyze Step (Optional)

Performs core evaluation analysis, gathering insights that will inform the scoring decision.

**Functions:** `({ run, results }) => any`

```typescript
const glutenCheckerScorer = createScorer({...})
.preprocess(...)
.analyze(({ run, results }) => {
  const { recipeText, hasCommonGlutenWords } = results.preprocessStepResult;

  // Simple gluten detection algorithm
  const glutenKeywords = ['wheat', 'flour', 'barley', 'rye', 'bread'];
  const foundGlutenWords = glutenKeywords.filter(word =>
    recipeText.includes(word)
  );

  return {
    isGlutenFree: foundGlutenWords.length === 0,
    detectedGlutenSources: foundGlutenWords,
    confidence: hasCommonGlutenWords ? 0.9 : 0.7
  };
})
```

**Prompt Objects:** Use `description`, `outputSchema`, and `createPrompt` for LLM-based analysis.

```typescript
const glutenCheckerScorer = createScorer({...})
.preprocess(...)
.analyze({
  description: 'Analyze recipe for gluten content',
  outputSchema: z.object({
    isGlutenFree: z.boolean(),
    glutenSources: z.array(z.string()),
    confidence: z.number().min(0).max(1)
  }),
  createPrompt: ({ run, results }) => `
    Analyze this recipe for gluten content:
    "${results.preprocessStepResult.recipeText}"

    Look for wheat, barley, rye, and hidden sources like soy sauce.
    Return JSON with isGlutenFree, glutenSources array, and confidence (0-1).
  `
})
```

**Data Flow:** Results are available to subsequent steps as `results.analyzeStepResult`

#### generateScore Step (Required)

Converts analysis results into a numerical score. This is the only required step in the pipeline.

**Functions:** `({ run, results }) => number`

```typescript
const glutenCheckerScorer = createScorer({...})
.preprocess(...)
.analyze(...)
.generateScore(({ results }) => {
  const { isGlutenFree, confidence } = results.analyzeStepResult;

  // Return 1 for gluten-free, 0 for contains gluten
  // Weight by confidence level
  return isGlutenFree ? confidence : 0;
})
```

**Prompt Objects:** See the [`createScorer`](/reference/v1/evals/create-scorer) API reference for details on using prompt objects with generateScore, including required `calculateScore` function.

**Data Flow:** The score is available to generateReason as the `score` parameter

#### generateReason Step (Optional)

Generates human-readable explanations for the score, useful for debugging, transparency, or user feedback.

**Functions:** `({ run, results, score }) => string`

```typescript
const glutenCheckerScorer = createScorer({...})
.preprocess(...)
.analyze(...)
.generateScore(...)
.generateReason(({ results, score }) => {
  const { isGlutenFree, glutenSources } = results.analyzeStepResult;

  if (isGlutenFree) {
    return `Score: ${score}. This recipe is gluten-free with no harmful ingredients detected.`;
  } else {
    return `Score: ${score}. Contains gluten from: ${glutenSources.join(', ')}`;
  }
})
```

**Prompt Objects:** Use `description` and `createPrompt` for LLM-generated explanations.

```typescript
const glutenCheckerScorer = createScorer({...})
.preprocess(...)
.analyze(...)
.generateScore(...)
.generateReason({
  description: 'Explain the gluten assessment',
  createPrompt: ({ results, score }) => `
    Explain why this recipe received a score of ${score}.
    Analysis: ${JSON.stringify(results.analyzeStepResult)}

    Provide a clear explanation for someone with dietary restrictions.
  `
})
```

## Example: Create a custom scorer

A custom scorer in Mastra uses `createScorer` with four core components:

1. [**Judge Configuration**](#judge-configuration)
2. [**Analysis Step**](#analysis-step)
3. [**Score Generation**](#score-generation)
4. [**Reason Generation**](#reason-generation)

Together, these components allow you to define custom evaluation logic using LLMs as judges.

> See [createScorer](/reference/v1/evals/create-scorer) for the full API and configuration options.

```typescript title="src/mastra/scorers/gluten-checker.ts" showLineNumbers copy
import { createScorer } from "@mastra/core/evals";
import { z } from "zod";

export const GLUTEN_INSTRUCTIONS = `You are a Chef that identifies if recipes contain gluten.`;

export const generateGlutenPrompt = ({
  output,
}: {
  output: string;
}) => `Check if this recipe is gluten-free.

Check for:
- Wheat
- Barley
- Rye
- Common sources like flour, pasta, bread

Example with gluten:
"Mix flour and water to make dough"
Response: {
  "isGlutenFree": false,
  "glutenSources": ["flour"]
}

Example gluten-free:
"Mix rice, beans, and vegetables"
Response: {
  "isGlutenFree": true,
  "glutenSources": []
}

Recipe to analyze:
${output}

Return your response in this format:
{
  "isGlutenFree": boolean,
  "glutenSources": ["list ingredients containing gluten"]
}`;

export const generateReasonPrompt = ({
  isGlutenFree,
  glutenSources,
}: {
  isGlutenFree: boolean;
  glutenSources: string[];
}) => `Explain why this recipe is${isGlutenFree ? "" : " not"} gluten-free.

${glutenSources.length > 0 ? `Sources of gluten: ${glutenSources.join(", ")}` : "No gluten-containing ingredients found"}

Return your response in this format:
"This recipe is [gluten-free/contains gluten] because [explanation]"`;

export const glutenCheckerScorer = createScorer({
  name: "Gluten Checker",
  description: "Check if the output contains any gluten",
  judge: {
    model: "openai/gpt-4.1-nano",
    instructions: GLUTEN_INSTRUCTIONS,
  },
})
  .analyze({
    description: "Analyze the output for gluten",
    outputSchema: z.object({
      isGlutenFree: z.boolean(),
      glutenSources: z.array(z.string()),
    }),
    createPrompt: ({ run }) => {
      const { output } = run;
      return generateGlutenPrompt({ output: output.text });
    },
  })
  .generateScore(({ results }) => {
    return results.analyzeStepResult.isGlutenFree ? 1 : 0;
  })
  .generateReason({
    description: "Generate a reason for the score",
    createPrompt: ({ results }) => {
      return generateReasonPrompt({
        glutenSources: results.analyzeStepResult.glutenSources,
        isGlutenFree: results.analyzeStepResult.isGlutenFree,
      });
    },
  });
```

### Judge Configuration

Sets up the LLM model and defines its role as a domain expert.

```typescript
judge: {
  model: 'openai/gpt-4.1-nano',
  instructions: GLUTEN_INSTRUCTIONS,
}
```

### Analysis Step

Defines how the LLM should analyze the input and what structured output to return.

```typescript
.analyze({
  description: 'Analyze the output for gluten',
  outputSchema: z.object({
    isGlutenFree: z.boolean(),
    glutenSources: z.array(z.string()),
  }),
  createPrompt: ({ run }) => {
    const { output } = run;
    return generateGlutenPrompt({ output: output.text });
  },
})
```

The analysis step uses a prompt object to:

- Provide a clear description of the analysis task
- Define expected output structure with Zod schema (both boolean result and list of gluten sources)
- Generate dynamic prompts based on the input content

### Score Generation

Converts the LLM's structured analysis into a numerical score.

```typescript
.generateScore(({ results }) => {
  return results.analyzeStepResult.isGlutenFree ? 1 : 0;
})
```

The score generation function takes the analysis results and applies business logic to produce a score. In this case, the LLM directly determines if the recipe is gluten-free, so we use that boolean result: 1 for gluten-free, 0 for contains gluten.

### Reason Generation

Provides human-readable explanations for the score using another LLM call.

```typescript
.generateReason({
  description: 'Generate a reason for the score',
  createPrompt: ({ results }) => {
    return generateReasonPrompt({
      glutenSources: results.analyzeStepResult.glutenSources,
      isGlutenFree: results.analyzeStepResult.isGlutenFree,
    });
  },
})
```

The reason generation step creates explanations that help users understand why a particular score was assigned, using both the boolean result and the specific gluten sources identified by the analysis step.

````

## High gluten-free example

```typescript title="src/example-high-gluten-free.ts" showLineNumbers copy
const result = await glutenCheckerScorer.run({
  input: [{ role: 'user', content: 'Mix rice, beans, and vegetables' }],
  output: { text: 'Mix rice, beans, and vegetables' },
});

console.log('Score:', result.score);
console.log('Gluten sources:', result.analyzeStepResult.glutenSources);
console.log('Reason:', result.reason);
````

### High gluten-free output

```typescript
{
  score: 1,
  analyzeStepResult: {
    isGlutenFree: true,
    glutenSources: []
  },
  reason: 'This recipe is gluten-free because rice, beans, and vegetables are naturally gluten-free ingredients that are safe for people with celiac disease.'
}
```

## Partial gluten example

```typescript title="src/example-partial-gluten.ts" showLineNumbers copy
const result = await glutenCheckerScorer.run({
  input: [{ role: "user", content: "Mix flour and water to make dough" }],
  output: { text: "Mix flour and water to make dough" },
});

console.log("Score:", result.score);
console.log("Gluten sources:", result.analyzeStepResult.glutenSources);
console.log("Reason:", result.reason);
```

### Partial gluten output

```typescript
{
  score: 0,
  analyzeStepResult: {
    isGlutenFree: false,
    glutenSources: ['flour']
  },
  reason: 'This recipe is not gluten-free because it contains flour. Regular flour is made from wheat and contains gluten, making it unsafe for people with celiac disease or gluten sensitivity.'
}
```

## Low gluten-free example

```typescript title="src/example-low-gluten-free.ts" showLineNumbers copy
const result = await glutenCheckerScorer.run({
  input: [{ role: "user", content: "Add soy sauce and noodles" }],
  output: { text: "Add soy sauce and noodles" },
});

console.log("Score:", result.score);
console.log("Gluten sources:", result.analyzeStepResult.glutenSources);
console.log("Reason:", result.reason);
```

### Low gluten-free output

```typescript
{
  score: 0,
  analyzeStepResult: {
    isGlutenFree: false,
    glutenSources: ['soy sauce', 'noodles']
  },
  reason: 'This recipe is not gluten-free because it contains soy sauce, noodles. Regular soy sauce contains wheat and most noodles are made from wheat flour, both of which contain gluten and are unsafe for people with gluten sensitivity.'
}
```

**Examples and Resources:**

- [createScorer API Reference](/reference/v1/evals/create-scorer) - Complete technical documentation
- [Built-in Scorers Source Code](https://github.com/mastra-ai/mastra/tree/main/packages/evals/src/scorers) - Real implementations for reference
